free thyroxine

thyroxine

(T4) [thi-rok´sin]

a thyroid hormone that contains iodine and is a derivative of the amino acid tyrosine, occurring naturally as l-thyroxine; its chemical name is tetraiodothyronine. It is formed and stored in the thyroid follicles as thyroglobulin and released from the gland by the action of a proteolytic enzyme. It is deiodinated in peripheral tissues to form triiodothyronine (T3), which has a greater biological activity.

Thyroxine acts as a catalyst in the body and influences a great variety of effects, including metabolic rate (oxygen consumption); growth and development; metabolism of carbohydrates, fats, proteins, electrolytes, and water; vitamin requirements; reproduction; and resistance to infection. Pharmaceutical preparations of thyroxine, extracted from animals or made synthetically, are called levothyroxine.

free thyroxine the fraction of thyroxine in the serum that is not bound to a binding protein.

free thyroxine

the amount of the unbound, active thyroid hormone thyroxine circulating in the blood, measured by specific laboratory procedures. See also free thyroxine index.

thyroxine, thyroxin

a hormone of the thyroid gland that contains iodine and is a derivative of the amino acid tyrosine. The chemical name for thyroxine is tetraiodothyronine (symbol, T4); it is formed and stored in the thyroid follicles as thyroglobulin, the storage form. Thyroxine is released from the gland by the action of a proteolytic enzyme. T4 is deiodinated in peripheral tissues to form tri-iodothyronine (T3), which has a greater biological activity.

Thyroxine acts as a catalyst in the body and influences a great variety of effects, including metabolic rate (oxygen consumption); growth and development; metabolism of carbohydrates, fats, proteins, electrolytes and water; vitamin requirements; reproduction; and resistance to infection.

Thyroxine can be extracted from animals or made synthetically; it is used in the treatment of hypothyroidism and some types of goiter.

free thyroxine

the metabolically active fraction of thyroxine; abbreviated FT4, Tf. T4 = Tf + TBG. The amount is very small and difficult to estimate so that the amount present in serum is not used as a more accurate indicator of thyroxine status than T4.

thyroxine-binding globulin (TBG)

most (99.95%) of the thyroxine in plasma is bound to globulin and a small amount bound to prealbumin.

thyroxine-binding prealbumin

bound to a small fraction of circulating T4. This is the only known function of prealbumins.

More specifically, a low free thyroxine at 2 and 4 weeks was associated with poorer attention skills at 3 months of corrected age as assessed by the Bayley Scales of Infant Development, Second Edition.

0 mIU/L and normal free thyroxine (T4) and free triiodothyronine (T3) levels in the blood -- the standard definition of subclinical hypothyroidism -- had no significant differences in cognitive function and hypothyroid symptoms when compared to a healthy control group.

The need to measure free thyroxine levels has diminished in the past 2 decades, but it's still a frequently ordered test and may be an important one for some patient populations, especially pregnant women.

All content on this website, including dictionary, thesaurus, literature, geography, and other reference data is for informational purposes only. This information should not be considered complete, up to date, and is not intended to be used in place of a visit, consultation, or advice of a legal, medical, or any other professional.